Power supply with capacitive mains isolation

ABSTRACT

A power supply with capacitive mains isolation, comprises a source of an input supply voltage developed between a first terminal and a second terminal. A first supply inductance is coupled to the first terminal, and a second supply inductance is coupled to a load circuit. A switch responsive to a periodic control signal applies the input supply voltage to the first supply inductance to generate a current in the first supply inductance at a first polarity, during a first portion of a period of the control signal when the switch is at a first switch state. A pair of capacitors operate to couple the first supply inductance to the second supply inductance during a second portion of the period of the control signal when the switch is at a second switch state. The pair of capacitors isolate the first and second terminals, respectively, from the second supply inductance at a range of frequencies that is lower than a frequency of the control signal. A first rectifier is coupled to the first supply inductance for preventing the first supply inductance current from changing polarity, during the second portion of the period.

RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 ofProvisional Patent Application Ser. No. 60/418,823 filed on Oct. 16,2002.

FIELD OF THE INVENTION

The present invention relates to power supplies in general, and moreparticularly, to generating a supply voltage using capacitive isolation.

BACKGROUND OF THE INVENTION

Power supplies serve the purpose of converting an input voltage into oneor several output voltages. An AC power source may be used to provide anAC power line input, which gets converted to a DC regulated outputvoltage. Transformers are typically used to provide isolation for apower supply or a converter. However, transformers are typically largein size (due to the size of the magnetic elements within them), bulkyand expensive devices. A regulated power supply that utilizes capacitiveelements to transform an input voltage from an AC power source to aspecified output voltage level across a load is desired.

SUMMARY OF THE INVENTION

A power supply with capacitive mains isolation, comprises a source of aninput supply voltage developed between a first terminal and a secondterminal. A first supply inductance is coupled to the first terminal,and a second supply inductance is coupled to a load circuit. A switchresponsive to a periodic control signal applies the input supply voltageto the first supply inductance to generate a current in the first supplyinductance at a first polarity, during a first portion of a period ofthe control signal when the switch is at a first switch state. A pair ofcapacitors operate to couple the first supply inductance to the secondsupply inductance during a second portion of the period of the controlsignal when the switch is at a second switch state. The pair ofcapacitors isolate the first and second terminals, respectively, fromthe second supply inductance at a range of frequencies that is lowerthan a frequency of the control signal. A first rectifier is coupled tothe first supply inductance for preventing the first supply inductancecurrent from changing polarity, during the second portion of the period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a power supply with capacitive mains isolation inaccordance with an embodiment of the present invention.

FIG. 2 shows waveforms associated with the operation of the circuitshown in FIG. 1.

FIG. 3 shows a standby power supply with capacitive mains isolation inaccordance with another embodiment of the present invention.

FIG. 4 shows a standby power supply with capacitive mains isolationincluding a mechanism for providing a lower output voltage in accordancewith another embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 depicts an exemplary circuit arrangement 100 adapted to provide apower supply having capacitive mains isolation according to anembodiment of the present invention. An input supply voltage Vin isdeveloped and applied at input node 10 a of circuit 100. Inductor L1 anddiode D1 are connected in series with one another, with L1 having afirst terminal L1 a connected to node 10 a, and a second terminal L1 bconnected to the anode of diode D1. The cathode of diode D1 is connectedat node 10 b to terminal C1 a of first isolation capacitor C1, whosesecond terminal C1 b is connected to node 10 c. Switch S1 selectivelyprovides a direct path between node 10 b and reference potential orground (GND). Isolation capacitor C2 has a first terminal connected toGND and a second terminal connected to C1 through inductor L2.Capacitors C1 and C2 provide isolation due to the fact that thecapacitors have a high impedance at the relatively low frequency of Vin.However, the capacitors represent a low impedance at the frequency ofoperation of switch S1, which is at a higher frequency than that of Vin.Switch S1 is responsive to a control signal 62 from control circuit 60for selectively opening/closing the connection between node 10 b and GNDto disable/enable application of the input supply voltage Vin toinductor L1. In this manner the switch is operated at a given frequencyf1 in accordance with the control signal. Capacitors C1 and C2 have lowimpedance with respect to this frequency. For example, capacitors C1 andC2 may have a low impedance in relation to operation of switch S1 at 50KHz, while providing a high impedance and isolation at an input voltageVin of, for example 50 Hz or 60 Hz. While switch S1 is shown as a simpleswitch, it is understood that various implementations are possible, suchas one or more switching transistors, relays, solenoids, and the like.

Still referring to FIG. 1, diode D2 is connected between nodes 10 c and10 d, while load RL and filter capacitor C3 are connected in parallelbetween nodes 10 d and 10 e. Isolation capacitor C2 has a first terminalC2 a coupled to GND, and a second terminal C2 b coupled to node 10 e.Inductor L2 is connected between node 10 c and 10 e. Node 10 e isconnected to a second reference or isolated ground potential gnd2.

Operation of power supply circuit 100 is described with reference toFIG. 1 in conjunction with the respective waveforms shown in FIG. 2.Initially (i.e. time to), switch S1 is closed (“on”) to cause currentI(L1) to rise linearly and store energy in inductor L1. At the same timea sinusoidal current I(L2) begins to transfer the stored energy of C1and C2 to L2. When the voltage across capacitor C1 exceeds the outputvoltage (Vout) at time t1, diode D2 conducts and the energy in inductorL2 is transferred to capacitor C3. Note that between time t0 and timet1, current flows in the path of circuit elements L2, C1, S1 and C2 (inthe counterclockwise direction). At time t1, the path of current changesto flow in the clockwise direction through circuit elements L2, D2 andC3. This is because the voltage across C1 and C2 equals the voltageacross C3 (i.e. Vout). At time t2, S1 is switched “off” (i.e. opened).Current I(L1) continues to flow through the circuit elements D1, C1, D2,C3 and C2 back to the mains until the energy stored in L1 is transferredto C1, C2 and C3. At time t2, current I(L1) slightly increases due tothe condition that Vin is higher than the voltage V(S1) at node 10 b.During this energy transfer, diode D1 embodying an inventive feature isconducting in the forward direction. As soon as the current in L1 dropsto zero at time t3 (due to resonance with C1, C2, C3) diode D1 changesto a non-conductive state and the current I(L1) is maintained at zero. Aflow back of energy from C1, C2 back to L1 (oscillation) is therebyadvantageously suppressed. Diode D1 advantageously increases theefficiency of the circuit since the entire energy is thereby stored inC1 and C2. Diode D1 allows the use of a free-running oscillator forcontrolling S1. At time t4, the energy in L2 is completely transferredinto C3. The time interval between t4 and t5 is a “dead time” and can bevaried to regulate the transfer of power between the input and outputterminals. Note that when S1 is again turned “on” (e.g. at time t5),current I(L1) again rises linearly, and the above described sequence isrepeated.

FIG. 3 illustrates an exemplary embodiment of a standby power supplyhaving capacitive mains isolation according to the present invention. Acontrol unit U1 such as a standard SMPS controller, is connected betweennodes 10 b and GND via terminals U1 d and U1 g, respectively, andfurther includes a supply terminal U1 a and feedback terminal U1 b. U1 ais the supply terminal where the internal supply voltage of theintegrated circuit (internal to U1) is developed. More particularly, thesupply terminal U1 a is energized internally in the IC in a manner notshown. Bypass capacitor C4 is connected between supply terminal U1 a andGND for providing power from integrated circuit U1, while terminal U1 bis connected to coupler U2, which may be an opto-coupler, for example.In an exemplary embodiment, control unit U1 operates at a constantfrequency, such as 50 Kilohertz (50 KHz) and includes an internalstartup or initialization circuit. Voltage regulation is realized byoperating the control unit in an on/off mode or burst mode. When theoutput voltage Vout exceeds the voltage reference level of Zener diodeD4 which is connected to secondary ground gnd2 by resistor R1, thecontroller is switched off via opto-coupler U2. A decreasing outputvoltage turns the controller on again. Note that L2 can be provided witha tap T as shown in FIG. 4, in the event that a second, lower outputvoltage is desired. As illustrated in the embodiment of FIG. 4, tap T isconnected to inductor L2, and to capacitor C4 via diode D5 to secondaryground gnd2. Note that the output voltage VLS is provided by the ratioof the turns of the inductor L2.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. The appended claims should beconstrued broadly to include other variants and embodiments of theinvention which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

1. A power supply comprising: a source of an input supply voltagedeveloped between a pair of input supply terminals; a first supplyinductance; a switch responsive to a periodic control signal forproducing from said input supply voltage a periodic current at a firstfrequency in said first supply inductance that is non-isolated from eachof said first pair of terminals; a pair of capacitors coupled in acurrent path of said first supply inductance current for developing anoutput supply between a pair of output supply terminals that are coupledto a load, said pair of capacitors providing capacitive isolation ofsaid output supply terminals from said input supply terminals,respectively, at frequencies lower than said first frequency and formwith said first supply inductance a first resonant circuit that variessaid first supply inductance current in a resonant manner operation,during a first portion of a period of said first supply inductancecurrent; and a first rectifier coupled in said current path forpreventing the first supply inductance current variation from resultingin a polarity change of said first supply inductance current, duringsaid first period portion.
 2. The power supply according to claim 1,further comprising a second supply inductance coupled between said pairof capacitors, wherein said first supply inductance current charges saidpair of capacitors in a first direction, during said first portionperiod, and wherein said second supply inductance forms with said pairof capacitors a second resonant circuit for charging said pair ofcapacitors in an opposite direction, during a second portion of saidperiod.
 3. The power supply according to claim 2, wherein said switch isat a first state, during said first period portion, and at a secondstate, during said second period portion.
 4. The power supply accordingto claim 2, further comprising a second rectifier for rectifying avoltage developed in said second supply inductance to develop arectified output supply voltage in said load.
 5. The power supplyaccording to claim 4, further comprising a filter capacitor for saidrectified output supply voltage to form a low impedance path betweensaid pair of capacitors, during said first portion of said period. 6.The power supply of claim 1, wherein the first rectifier comprises adiode having a first terminal connected to the first inductance, and asecond terminal connected to one of the pair of capacitors.
 7. The powersupply of claim 6, further comprising a second rectifier coupled betweenthe first one of the capacitors and the load.
 8. The power supply ofclaim 1, wherein the periodic control signal is produced from anoscillator circuit coupled to the switch.
 9. The power supply of claim1, wherein the switch comprises a switching transistor.
 10. The powersupply of claim 1, further comprising a third capacitor connected inparallel with the load.
 11. The power supply of claim 1, wherein theperiodic control signal is produced from a control circuit coupled tothe switch and operated in one of an on/off and burst mode of operation.12. The power supply of claim 11, wherein an opto-coupler is connectedto the control circuit such that when an output voltage across the loadexceeds a reference voltage, the controller is switched off via theopto-coupler.
 13. The power supply of claim 12, further comprising adiode in parallel with the load for sensing when an output voltageacross the load exceeds the reference voltage.
 14. The power supply ofclaim 1, further comprising a second supply inductance coupled betweensaid pair of capacitors and a tap coupled to a second supply inductancefor providing a second lower output supply voltage.
 15. A power supply,comprising: a source of an input supply voltage developed between a pairof input supply terminals; a first supply inductance; a switchresponsive to a periodic control signal for producing from said inputsupply voltage a periodic current at a first frequency in said firstsupply inductance that is non-isolated from each of said first pair ofterminals; a second supply inductance; a pair of capacitors coupled inseries with said first supply inductance, during a first portion of aperiod of said control signal, and having said second supply inductancecoupled between said capacitors for developing from a voltage producedin said second supply inductance an output supply between a pair ofoutput supply terminals of a load, said pair of capacitors providingcapacitive isolation of said output supply terminals from said inputsupply terminals at frequencies lower than said first frequency; and afirst rectifier coupled in series with said first supply inductance forpreventing said first supply inductance current from changing polarity.